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From: "thiago at kde dot org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/103069] cmpxchg isn't optimized
Date: Wed, 03 Nov 2021 20:53:13 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103069

--- Comment #1 from Thiago Macieira <thiago at kde dot org> ---
(the assembly doesn't match the source code, but we got your point)

Another possible improvement for the __atomic_fetch_{and,nand,or} functions=
 is
that it can check whether the fetched value is already correct and branch o=
ut.
In your example, the __atomic_fetch_or with 0x40000000 can check if that bi=
t is
already set and, if so, not execute the CMPXCHG at all.

This is a valid solution for x86 on memory orderings up to acq_rel. For oth=
er
architectures, they may still need barriers. For seq_cst, we either need a
barrier or we need to execute the CMPXCHG at least once.=20

Therefore, the emitted code might want to optimistically execute the operat=
ion
once and, if it fails, enter the load loop. That's a slightly longer codege=
n.
Whether we want that under -Os or not, you'll have to be the judge.

Prior art: glibc/sysdeps/x86_64/nptl/pthread_spin_lock.S:
ENTRY(__pthread_spin_lock)
1:      LOCK
        decl    0(%rdi)
        jne     2f
        xor     %eax, %eax
        ret

        .align  16
2:      rep
        nop
        cmpl    $0, 0(%rdi)
        jg      1b
        jmp     2b
END(__pthread_spin_lock)

This does the atomic operation once, hoping it'll succeed. If it fails, it
enters the PAUSE+CMP+JG loop until the value is suitable.=